Applications of thin film transistors (TFT's) to image display elements of a flat panel display or the like have been actively developed in recent years. Of all types of thin film transistors, those employed in an active matrix type liquid crystal display or the like must meet the following demands: high mobility, high ON/OFF-state current ratio, resistance to high voltage, downsized elements, etc.
Polycrystalline semiconductor TFT's are advantageous over those comprising amorphous semiconductor films in terms of large conductance. However, the polycrystalline semiconductor TFT's have a drawback that their processing temperature is as high as 1000.degree. C. Thus, crystallization technology using a laser anneal technique has been actively studied or applied to obtain a polycrystalline semiconductor film at a processing temperature of 600.degree. C. or below.
FIG. 12 is a bottom view showing a part of a panel substrate of a liquid crystal display device having a plurality of conventional thin film transistors 100's comprising polycrystalline semiconductor films. FIG. 13 is a cross section taken on line E--E of FIG. 12, and FIGS. 14(a) through 14(h) show a fabrication sequence of the thin film transistor 100.
In the fabrication sequence of the thin film transistor 100, as shown in FIG. 14(a), an amorphous semiconductor film (.alpha.-Si) 102a is formed on a glass substrate 101 to begin with, and the amorphous semiconductor film 102a is developed into a polycrystalline semiconductor film 102b, through, for example, irradiation of an excimer laser beam (FIG. 14(b)). Then, the polycrystalline semiconductor film 102b is formed into a predetermined pattern (FIG. 14(c)). Next, a mask is made out of a photoresist film 103 on the polycrystalline semiconductor film 102b over a portion which will be made into a channel region, and impurity ions are doped into the polycrystalline semiconductor film 102b (ion doping) using the mask of the photoresist film 103 as a doping mask (ion implantation mask) (FIG. 14(d)). Then, after the photoresist film 103 is removed, the ions are activated and diffused through, for example, irradiation of an excimer laser beam to form a source region 104a and a drain region 105a (FIG. 14(e)). Next, a gate dielectric film 106 and a metal film are formed sequentially, and the metal film is made into a predetermined pattern to form a gate electrode 107 (FIG. 14(f)). After an interlayer dielectric film 108 is formed, the interlayer dielectric film 108, together with the gate dielectric film 106, is formed into a predetermined pattern to make contact holes 109's (FIG. 14(g)). Then, another metal film is formed and the same is made into a predetermined pattern to form a source electrode 104 and a drain electrode 105 (FIG. 14(h)). Further, a pixel electrode 110 is formed near the transistor thin film 100 thus fabricated in such a manner to have physical contact with the drain electrode 105 (FIGS. 12 and 13). The pixel electrode 110 is made of a transparent conductive film, such as an ITO film.
In the field of the liquid crystal display devices, a technique such that forms an auxiliary capacitance in parallel to a liquid crystal capacitance in each pixel is known to improve data withholding characteristics. To be more specific, this is a technique to minimize a voltage drop by connecting the auxiliary capacitance electrode to the thin film transistor in series, while connecting the same to the capacitance of the liquid crystal cell in parallel. A technique to make either electrode of the auxiliary capacitance out of polysilicon is described in pages 387-390, SID1993DIGEST.
To save the costs of fabricating the thin film transistors, various methods have been proposed to simplify the fabrication sequence, and an example of which is disclosed in Japanese Laid-open Patent Application No. 5-235031 (1993). In the reference, as shown in FIG. 15(a), an amorphous semiconductor film 102a is formed on a glass substrate 101 to begin with, and a mask is made out of a photoresist film 103 on the amorphous semiconductor film 102a over a portion which will be made into a channel region. Then, impurity ions are doped into the amorphous semiconductor film 102a using the mask of the photoresist film 103 as a doping mask to form impurity doped regions 111's (FIG. 15(b)). After the photoresist film 103 is removed, for example, an excimer laser beam is irradiated so that the ion doped regions 111's are activated, and concurrently, the amorphous semiconductor film 102a is developed into a polycrystalline semiconductor film 102b (FIG. 15(c)). Then, The polycrystalline semiconductor film 102b is made into a predetermined pattern to form the channel region and the activated impurity doped regions 111's, which will be made into the channel region, a source region 104, and a drain region 105a, respectively (FIG. 15(d)). Next, a gate dielectric film 106 and a metal film are formed sequentially (FIG. 15(e)), and the metal film is made into a predetermined pattern to form a gate electrode 107. Similar steps to those explained with reference to FIGS. 14(f) through 14(h) are carried out thereafter to fabricate a thin film transistor. In the above fabrication process, the impurity ions may be doped into the amorphous semiconductor 102a through the ion implantation after the amorphous semiconductor film 102a is made into a predetermined pattern.
According to the above fabrication process, in the first place, the impurity is doped into the amorphous semiconductor 102a through the ion implantation to form the impurity doped regions 111's which will be made into the source region 104a and drain region 105a, respectively. In the second place, the amorphous semiconductor film 102a is polycrystalized and the impurity doped regions 111's are activated concurrently through irradiation of the excimer laser beam, for example. In short, both the polycrystalization and impurity activation are completed, for example, by irradiating an excimer laser beam once, thereby making it possible to simplify the fabrication sequence.
In either fabrication process above, the impurity doped regions are made first, and thence the gate electrode is formed on the upper layer of the channel region portion through the gate dielectric film.
However, in the practical process, the photomask becomes misaligned due to thermal contraction of the glass substrate or the like, and as shown FIG. 16, the gate electrode 107 may sometimes overlap the upper layer of the impurity doped region (source region 104a or drain region 105a) in the resulting thin film transistor. Such a thin film transistor characteristically attains a low breakdown voltage at the junction of the source portion or drain portion, and therefore, causes an increase in an OFF-state current. Thus, if such a thin film transistor is employed as a switching element of the pixel electrode of an active matrix type liquid crystal display device, there occurs display defects, such as a flicker.